The answer to your question would be that to create a parallel structure in the sentence, the phrase that would replace the words "when the plays are tragic" best is the following one: Plays that are tragic.
Parallel structure, also known as parallelism, is the repetition of a grammatical construction within in a sentence. In this case, the construction repetead is the whole NP (the noun head and its modifiers: plays that are funny and plays that are tragic)